1. Field of the Invention
The present invention relates to an image taking apparatus capable of producing a plurality of images of objects to which distances are mutually different.
2. Description of the Related Art
In the past, various proposals have been made for an imaging apparatus offering a large depth of field and capable of acquiring an image which is in focus relative to positions of a three-dimensional object, even though the depth of field of the optical system small.
A microscope having of a large focal length and a large depth of field, which is one of the proposed imaging apparatuses, is an optical microscope using a varifocal lens, the shape of which is changed quickly by changing the internal pressure of poured transparent fluid, which is used as an objective lens. The microscope takes advantage of the fact that a human being perceives an afterimage and accordingly varies the focal point of the varifocal lens at a high speed in order to display an image provides a large depth of field in real time.
However, the varifocal lens employed in the known microscope having of a large focal length and a large depth of field has the focal point thereof varied when the poured transparent fluid is externally pressurized. A piezoelectric device is used as an actuator for varying the focal point. A high-voltage amplifier capable of providing an output voltage of about 100 V is therefore needed as a driving means for driving the actuator. Moreover, a glass diaphragm of about 10 micrometers thick is used as an optical device. Since these special components are required, the cost of the lens itself or drive unit, mass-productivity, and reliability are unsatisfactory.
An image input/output apparatus disclosed in Japanese Unexamined Patent Publication No. 1-309478 and proposed previously by this applicant is designed to convert an object image acquired through an imaging lens into an electrical image signal by means of an imaging device. The focal point of the imaging lens is shifted by means of a focal point control unit in order to acquire object image information with respect to a plurality of screens, each of which depict objects that are spaced by a given distance and are in focus. The object image information constituting the plurality of screens is then added up and filtered. This results in video information representing the screens that depict the objects which are in focus, and provides a large depth of field.
The image input/output apparatus disclosed in the Japanese Unexamined Patent Publication No. 1-309478 uses normal lenses to construct an optical system. When the normal lenses are used, four or five lenses are needed for attaining brightness inversely proportional to, for example, a square of an f-number F2.8. This kind of lens itself is heavy and its lens frame is heavy. Several seconds are therefore required for producing a plurality of images by driving a motor to shift the focal point of the optical system. In other words, several seconds are needed for producing one image which provides a large depth of field. It is difficult to speed up the operations and display an image in real time because of the large weights of the lens and lens frame.
The image input apparatuses described in Japanese Examined Patent Publication No. 5-27084 and Germany Patent Publication No. DT2301800 are designed to increase a depth of field by shifting the focus of an optical system during an exposure time. For shifting the focus, the former moves either a sample or lens barrel in a microscope along the optical axis of the optical system, while the latter moves an object on a vibrating table, along the optical axis.
However, these methods suffer from a drawback that real-time display is restricted or cannot be achieved. For displaying an image, which provides a large depth of field, at the rate of about 60 frames per second, the focus of an optical system must be shifted by driving a lens barrel so as to vibrate it at a frequency of at least 30 Hz. However, it is difficult to drive the lens barrel of a microscope so as to vibrate it at this frequency. Moreover, when an attempt is made to drive the object so as to vibrate it, the attempt would succeed only when the object is small enough and lightweight. When the sample is large and heavy, such driving is impossible to do.
The present invention is directed at solving the foregoing drawbacks. An object of the present invention is to provide an image taking apparatus that can produce various images of an object lying along an optical axis at a high speed even when the optical system of the apparatus has of a relatively small depth of field. The present invention does not employ any special device, and has a simple configuration. Another object of the present invention is to provide an image taking apparatus capable of producing an image, which provides a relatively large depth of field, irrespective of the size and weight of a sample that is an object.
According to one aspect of the invention, there is provided an image taking apparatus of the present invention comprising: an optical system structured so that mutually-parallel different reflecting surfaces can be sequentially or concurrently defined in order to sequentially or concurrently define different focal points; an imaging device for forming object images, which are acquired by setting the optical system to the different focal points or acquired by shifting the focus of the optical system within a range, on the imaging surface thereof, and producing image information representing the object images; and a depth-of-field adjustment element for reconstructing an image equivalent to an image, which is produced by an image taking apparatus offering a larger depth of field than a depth of field attained with the optical system set to one focal point, according to the image information, produced by the imaging device, representing the object images acquired by setting the optical system to the different focal points or by shifting the focus of the optical system within a range.
In the foregoing aspect of the invention, a plurality of object images are acquired by dynamically focusing the optical system at points within a given range or acquired by statically and concurrently focusing it at a plurality of points. The object images are formed on an imaging surface in order to produce image information. Based on the image information thus acquired, an image providing a large depth of field is reconstructed.
Other features and advantages of the present invention will be fully apparent from the description below.